1. Field of the Invention
The invention relates to multiple speed automatic transmissions of the type used in motor vehicles. In particular, this invention pertains to a transmission that produces a low gear ratio that is suited for high torque, low vehicle speed conditions.
2. Description of the Prior Art
In a motor vehicle powertrain having four-wheel drive or all-wheel drive capability, an automatic transmission is driveably connected to an engine crankshaft and produces multiple ratios of the engine speed at a transmission output shaft, which is connected to the input of a transfer case. The transfer case generally includes a planetary gear unit for producing a high range and low range, which ranges potentially adjust the speed of the transfer case output relative to the speed at the transmission output. Usually the high range is a direct drive range in which the transfer case produces no speed reduction or torque amplification of the speed and torque at the transmission output. The low range amplifies the torque and decreases the speed of the transmission output. Operation in the low range is usually reserved for low speed, high torque or towing conditions; the high range is best suited for highway conditions.
In addition to producing both high and low speed ranges, the transfer case continually connects its output to a first set of vehicle wheels and selectively connects its output to a second set of wheels. Generally the vehicle must stop in order to change the range of operation of the transfer case into and out of the low range. When the low range is produced in the transfer case, the clutches that control the range-producing gear unit must be large in diameter and torque capacity in order to transmit the high torque associated with the low range.
It would be preferred that the transmission itself produce the low range speed ratio that conventionally is produced in the transfer case. If the transmission could produce the low-range ratio, the planetary gearing and friction elements associated with the range change in the transfer case could be eliminated. The controls required to coordinate transfer case operation with the requirements of the vehicle system could be located, at least in part, within the transmission and under control of the transmission control system. This would reduce transfer case complexity without unduly increasing the complexity of the transmission control system, or changing the nature or quality of the shift control normally provided by an automatic transmission control system.
If the transmission produced the low-range ratio, the low-range could be actuated on-the-fly, i.e., while the vehicle is moving under power, without stopping the vehicle to actuate and engage the low-range.